The present invention relates to a zoom lens system, and more particularly to a compact zoom lens system suitable for a lens shutter camera. The lens system is composed of five lens elements with features of high zoom ratio, short total length, and high quality.
Zoom lens systems are generally classified into two categories. The first category is the so-called retrofocus type zoom lens system, which typically consists of a front lens group having negative refractive power and a rear lens group having positive refractive power. An air separation is reserved between the front lens group and the rear lens group. It is possible to change the focal length of the lens system by varying the air separation between the two lens groups. The main feature of this lens type is that it has an enough space to arrange a reflection mirror, so that it is suitable to be used in a well-known single lens reflex camera. However, it has disadvantages in that the system has a long back focal length, thereby making it impossible to shorten the total length, and comprises the front lens having a large diameter, thereby making it impossible to make said zoom lens system compact.
The second category of zoom lens system is the so-called telephoto system which comprises a front lens group having positive refractive power and a rear lens group having negative refractive power. An airspace is also reserved between the front lens group and the rear lens group to vary the focal length of the lens system during zooming operation. This type of zoom lens system is characterized by a shorter back focal length, relative to the retrofocus type. The shorter back focal length makes it possible to shorten the total length of the zoom lens system. The telephoto lens system is especially suitable for use in a lens shutter camera.
It is intended to compact the zoom lens system to a smaller outside diameter, less lens element, and shorter total length. In addition, it is preferred to increase the zoom ratio of the lens system without decreasing the quality and making longer the total length of the lens system.
Various prior patents had been developed in this field. For example, U.S. Pat. No. 4,838,669 disclosed a zoom lens system having positive refractive power and a rear lens group having negative refractive power. The front lens group of the lens system consists of a positive meniscus lens, a negative lens, and at least one positive lens. The rear lens group of the lens system consists of a positive lens, a biconcave lens, and a negative meniscus lens.
U.S. Pat. No. 4,991,945 disclosed a zoom lens comprising five lens elements. The first three elements form a positive lens group and the last two elements form a negative lens group. In the front lens group, the first lens is a positive lens element, the second lens is a negative lens element, and the third lens is a positive lens element. In the rear lens group, the fourth lens is a positive lens element, and the fifth lens is a negative lens element.